Extruded solid dielectric cables installed underground over the last 15 years have been found to contain tree or bushlike patterns in the insulation. These patterns called "electrochemical trees" result from moisture penetration into the insulation under normal operating electrical stress due to high electrical stresses developed locally by insulation imperfections.
Under most operating conditions when extruded dielectric power cables are installed underground the ingress of moisture shortens the life of these cables prematurely because of the formation of these electrochemical trees in the insulation. These trees form at normal operating stress over a period of years depending somewhat on local environment and particular cable construction. The trees will not form if the insulation is initially dry or free of water and remains this way throughout its operating life. This condition can be achieved initially by an extruded lead sheath or other hermetic enclosure.
The trees represent channels which are filled with the liquid. The dielectric strength of the channels is lower than that of insulation. When the channels progress more than 50% throughout the insulation, the dielectric strength of the insulation is decreased to such an extent that electrical failure of the cable may occur under typical over-voltage conditions. Further progression of the channels into the insulation may cause failures at even operating voltages. The electrochemical trees are originated at insulation imperfections located within the insulation wall or at sharp irregularities at the interfaces of the insulation and the conductor and insulation shields. Typically, the electrochemical trees progress from high voltage stress areas caused by these imperfections into a low voltage stress area away from the imperfections.
As will be developed later, the purpose of this invention is to provide a special design of cable which, initially free of moisture in its components, is filled with moisture resistant materials to prevent sufficient moisture ingress and longitudinal movement during the active life of the cable to prematurely cause the cable to fail due to the formation of electrochemical trees.
Cables with extruded lead and aluminum sheaths have been used to prevent moisture ingress into the insulation and thereby retard the formation of electrochemical trees. Cables with welded sheaths, such as copper, aluminum and steel have also been used for the same purpose. However, these constructions have not been entirely satisfactory because moisture enters from the ends of the cable and at places where the sheath is damaged or has become corroded and progresses along the cable and into the insulation.
While these various expedients have been used to prevent moisture from getting into cables and destroying the effectiveness of certain types of insulation, such cable designs have not been effective in preventing the formation of electrochemical trees in the insulation over a period of years due to corrosion and damage to the metallic shield. In addition, metallic sheathed cables are high in cost, difficult to handle when large in diameter, and expensive to splice and terminate. This invention provides a combination of moisture repellent features that prevent or greatly delay the formation of electrochemical trees in power cable insulation and thereby extends substantially the useful life of the cable.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.